The invention relates to a process for the continuous preparation of melt processable polyurethanes in a static mixer with improved softening behaviour.
Thermoplastic polyurethane elastomers (TPU) are by no means new. They are of industrial importance in view of the combination of high-quality mechanical properties and the well known advantages of inexpensive melt processability. Due to the use of different chemical constituents, a wide variation of mechanical properties may be obtained. A review of TPUs, their properties and applications, is given, e.g., in Kunststoffe 68 (1978), pages 819 to 825 or Kautschuk, Gummi, Kunststoffe 35 (1982), pages 568 to 584.
TPUs are synthesised from linear polyols, mostly polyester or polyether polyols, organic diisocyanates and short-chain diols (chain extenders). In addition, catalysts may be added to accelerate the formation reaction. In order to adjust the properties, the constituents may be varied in relatively wide molar ratios. Molar ratios of polyols to chain extenders from 1:1 to 1:12 have proved suitable. As a result, products ranging from 70 Shore A to 75 Shore D are obtained.
The synthesis of melt processable polyurethane elastomers may take place either in steps (prepolymer metering process) or by the simultaneous reaction of all the components in one step (one-shot metering process).
The TPUs may be prepared continuously or batchwise. The most well known industrial production processes are the belt process (GB-A 1 057 018) and the extruder process (DE-A 19 64 834, DE-A 23 02 564 and DE-A 20 59 570). In the extruder process, the starting materials are metered into a screw reactor where polyaddition takes place, and are converted to a uniform granular form. The extruder process is comparatively simple but has the disadvantage that the homogeneity of the products thus produced is not sufficient for many applications in view of the fact that mixing and reaction proceed simultaneously. In addition, the softening behaviour of the TPUs and the moulded articles produced from them is limited. TPUs which melt readily, of the kind used e.g. for hot melt films or sintered products, can be prepared only to a limited extent, if at all, by this process.
Moreover, preparation processes are known from the literature in which the starting materials are initially mixed in a mixing zone at low temperatures at which no poly-addition occurs, and then react together in a reaction zone which has the desired reaction temperature. The mixing and reaction zone is designed preferably as a static mixer.
In DE-A 28 23 762, homogeneous products are obtained by the xe2x80x9cone-shot processxe2x80x9d. In EP-A 747 409, metering takes place by the prepolymer process and homogenous TPUs with improved mechanical properties are obtained.
The object was, therefore, to provide a simple process with which it is possible to prepare homogeneous TPUs with improved softening behaviour in an inexpensive and technically simple manner.
Surprisingly, this object was achieved by preparing TPUs continuously in a static mixer, in which the entire TPU reaction is carried out substantially in the xe2x80x9cone-shot metering processxe2x80x9d, under special process conditions. Homogeneous TPU products with markedly better melting properties are obtained with this process.
The invention provides a process for the continuous preparation of melt processable, homogeneous polyurethane elastomers with improved softening behaviour, in which
one or more polyisocyanates (A) and
a mixture (B) having Zerewitinoff active hydrogen atoms of
B1) 1 to 85 equivalent %, based on the isocyanate groups in (A), of one or more compounds with on average at least 1.8 and at most 2.2 Zerewitinoff active hydrogen atoms per molecule and an average molecular weight {overscore (M)}n from 450 to 5000 g/mole,
B2) 15 to 99 equivalent %, based on the isocyanate groups in (A), of one or more chain extenders with on average at least 1.8 and at most 2.2 Zerewitinoff active hydrogen atoms per molecule and a molecular weight from 60 to 400 g/mole, and
0 to 20 wt. %, based on the total quantity of TPU, of further auxiliaries and additives (C),
wherein the components A) and B) are used in an NCO:OH ratio of 0.9:1 to 1.:1,
are homogeneously mixed in a static mixer at a shear rate of  greater than 500 secxe2x88x921 and  less than 50,000 secxe2x88x921 within a maximum of 1 second, the reaction mixture thus prepared is metered into an extruder, optionally via a second static mixer, and optionally auxiliaries and/or further components are incorporated, characterised in that the polyisocyanate (A) and the mixture (B) each have a temperature of  greater than 170xc2x0 C. and  less than 250xc2x0 C. the reaction takes place substantially in the first static mixer with a conversion of  greater than 90%, based on component A), and the reaction mixture leaves the first static mixer at a temperature of  greater than 240xc2x0 C. and  less than 350xc2x0 C.
Examples of suitable organic polyisocyanates (A) include aliphatic, cycloaliphatic, araliphatic, heterocyclic and aromatic diisocyanates, as described e.g. in Justus Liebigs Annalen der Chemie, 562, pages 75 to 136.
More specifically, examples include: aliphatic diisocyanates such as hexamethylene diisocyanate, cycloaliphatic diisocyanates such as isophorone diisocyanate, 1,4-cyclohexane diisocyanate, 1-methyl-2,4- and 2,6-cyclohexane diisocyanate and the corresponding isomer mixtures, 4,4xe2x80x2-2,4xe2x80x2- and 2,2xe2x80x2-dicyclohexylmethane diisocyanate and the corresponding isomer mixtures and aromatic diisocyanates such as toluene 2,4-diisocyanate, mixtures of toluene 2,4- and 2,6-diisocyanate, 4,4xe2x80x2-diphenylmethane diisocyanate, 2,4xe2x80x2-diphenylmethane diisocyanate and 2,2xe2x80x2-diphenylmethane diisocyanate, mixtures of 2,4xe2x80x2-diphenylmethane diisocyanate and 4,4xe2x80x2-diphenylmethane diisocyanate, urethane-modified liquid 4,4xe2x80x2-diphenylmethane diisocyanates and/or 2,4xe2x80x2-diphenylmethane diisocyanates, 4,4xe2x80x2-diisocyananatodiphenylethane-(1,2) and 1,5-naphthylene diisocyanate. Diphenyl-methane diisocyanate isomer mixtures with a 4,4xe2x80x2-diphenylmethane diisocyanate content of more than 96 wt. % and in particular 4,4xe2x80x2-diphenylmethane diisocyanate and 1,5-naphthylene diisocyanate are used in preference. The diisocyanates mentioned may be used individually or in the form of mixtures. They may also be used together with up to 15% (based on total diisocyanate) but at most that amount of a polyisocyanate required to obtain a melt processable product. Examples are triphenylmethane-4,4xe2x80x24xe2x80x3-triisocyanate and polyphenylpolymethylene poly-isocyanates.
Linear hydroxyl-terminated polyols with on average 1.8 to 3.0, preferably to 2.2 Zerewitinoff active hydrogen atoms per molecule and with a molecular weight from 450 to 5000 g/mole are used as component B1). Due to production conditions, said polyols often contain small amounts of non-linear compounds. The term xe2x80x9csubstantially linear polyolsxe2x80x9d is often, therefore, used. Polyester, polyether, polycarbonate diols or mixtures thereof are preferred.
Suitable polyether diols may be prepared by reacting one or more alkylene oxides with 2 to 4 carbon atoms in the alkylene radical with a starter molecule which contains two active hydrogen atoms in the bound state. Examples of suitable alkylene oxides include: ethylene oxide, 1,2-propylene oxide, epichlorohydrin and 1,2-butylene oxide and 2,3-butylene oxide. Ethylene oxide, propylene oxide and mixtures of 1,2-propylene oxide and ethylene oxide are used in preference. The alkylene oxides may be used individually, in alternating succession or as mixtures. Examples of suitable starter molecules include: water, aminoalcohols such as N-alkyl diethanolamines, for example, N-methyl diethanolamine, and diols such as ethylene glycol, 1,3-propylene glycol, butane 1,4-diol and hexane 1,6-diol. Optionally, mixtures of starter molecules may also be used. Suitable polyetherols are also the hydroxyl group-containing polymerisation products of tetrahydrofuran. Trifunctional polyethers may also be used in proportions from 0 to 30 wt. %, based on the bifunctional polyethers, but at most in a quantity such that a melt processable product is obtained. The substantially linear polyether diols preferably have molecular weights from 450 to 5000 g/mole. They may be used both individually and in the form of mixtures.
Suitable polyester diols may be prepared, for example, from dicarboxylic acids with 2 to 12 carbon atoms, preferably 4 to 6 carbon atoms, and polyhydric alcohols. Examples of suitable dicarboxylic acids include: aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid and sebacic acid and aromatic dicarboxylic acids such as phthalic acid, isophthalic acid and terephthalic acid. The dicarboxylic acids may be used individually or as mixtures, e.g. in the form of a succinic, glutaric and adipic acid mixture. In order to prepare the polyester diols it may optionally be advantageous to use the corresponding dicarboxylic acid derivatives instead of the dicarboxylic acids, such as carboxylic acid diesters with 1 to 4 carbon atoms in the alcohol radical, carboxylic anhydrides or carboxylic acid chlorides. Examples of polyhydric alcohols are glycols with 2 to 10, preferably 2 to 6 carbon atoms such as ethylene glycol, diethylene glycol, butane 1,4-diol, pentane 1,5-diol, hexane 1,6-diol, decane 1,10-diol, 2,2-dimethyl-1,3-propane diol, propane 1,3-diol and dipropylene glycol. Depending on the properties required, the polyhydric alcohols may be used by themselves or optionally in mixture. Esters of carbonic acid with the diols mentioned are also suitable, particularly those with 4 to 6 carbon atoms, such as butane 1,4-diol or hexane 1,6-diol, condensation products of xcfx89-hydroxycarboxylic acids, for example, xcfx89-hydroxycaproic acid and preferably polymerisation products of lactones, for example, optionally substituted caprolactones. Polyester diols used in preference are ethane diol polyadipates, butane 1,4-diol polyadipates, ethane diol-butane-1,4-diol polyadipates, hexane 1,6-diol neopentylglycol polyadipates, hexane 1,6-diol-butane-1,4-diol polyadipates and polycaprolactones. The polyester diols have molecular weights from 450 to 5000 g/mole and may be used individually or in the form of mixtures.
Diols or diamines with on average 1.8 to 3.0, preferably to 2.2 Zerewitinoff active hydrogen atoms per molecule and a molecular weight from 60 to 400 g/mole are used as component B2), preferably aliphatic diols with 2 to 14 carbon atoms such as, e.g., ethane diol, hexane 1,6-diol, diethylene glycol, dipropylene glycol and particularly butane 1,4-diol. Diesters of terephthalic acid with glycols with 2 to 4 carbon atoms are also, however, suitable, such as, e.g., terephthalic acid-bis-ethylene glycol or terephthalic acid-bis-butane 1,4-diol, hydroxyalkylene ethers of hydroquinone such as, e.g., 1,4-di(xcex2-hydroxyethyl)hydroquinone, ethoxylated bisphenols such as, e.g., 1,4-di(xcex2-hydroxyethyl)-bisphenol A, (cyclo)aliphatic diamines, such as, e.g., isophorone diamine, ethylene diamine, 1,2-propylene diamine, 1,3-propylene diamine, N-methylpropylene-1,3-diamine, N,Nxe2x80x2-dimethylethylene diamine and aromatic diamines such as, e.g., 2,4-toluene diamine and 2,6-toluene diamine, 3,5-diethyl-2,4-toluene diamine and/or 3,5-diethyl-2,6-toluene diamine and primary mono-, di-, tri- and/or tetraalkylsubstituted 4,4xe2x80x2-diaminodiphenylmethanes. Mixtures of the above-mentioned chain extenders may also be used. In addition, relatively small amounts of triols may also be added.
Moreover, conventional monofunctional compounds may also be used in small amounts, e.g., as chain terminators or release agents. Examples include alcohols such as octanol and stearyl alcohol or amines such as butylamine and stearylamine.
In order to prepare the TPUs, the constituents, optionally in the presence of catalysts, auxiliaries and/or additives, may be reacted preferably in quantities such that the equivalent ratio of NCO groups A) to the sum of the NCO- reactive groups, particularly the OH groups of the low molecular weight diols/triols B2) and polyols B1) is 0.9:1.0 to 1.1:1.0, preferably 0.95:1.0 to 1.10:1.0.
Suitable catalysts according to the invention are conventional tertiary amines well known according to the state of the art, such as, e.g., triethylamine, dimethylcyclohexylamine, -methylmorpholine, N,Nxe2x80x2-dimethylpiperazine, 2-(dimethylaminoethoxy)ethanol, diazabicyclo-(2,2,2)-octane and the like, and in particular organic metal compounds such as titanates, iron compounds, tin compounds, e.g. tin diacetate, tin dioctoate, tin dilaurate or the tin dialkyl salts of aliphatic carboxylic acids such as dibutyltin diacetate, dibutyltin dilaurate or the like. Preferred catalysts are organic metal compounds, particularly titanates, iron and/or tin compounds.
Apart from the TPU components and the catalysts, auxiliaries and/or additives (C) may also be added in a quantity of up to 20 wt. %, based on the total quantity of TPU. They may be predissolved in one of the TPU components, preferably in component B1), or optionally metered in after reaction has taken place in a downstream mixing device, e.g., an extruder.
Examples include lubricants such as fatty acid esters, the metal soaps thereof, fatty acid amides, fatty acid ester amides and silicone compounds, antiblocking agents, inhibitors, stabilisers against hydrolysis, light, heat and discoloration, flame retardants, colorants, pigments, inorganic and/or organic fillers and reinforcing agents. Reinforcing agents are, in particular, fibre-like reinforcing agents such as, e.g., inorganic fibres which are produced according to the state of the art and may also be provided with a size. Further details about the auxiliaries and additives mentioned can be obtained from the technical literature, for example, the monograph of J. H. Saunders and K. C. Frisch: xe2x80x9cHigh Polymersxe2x80x9d, Vol. XVI, Polyurethane, Part 1 and 2, Verlag Interscience Publishers 1962 and 1964, Taschenbuch fxc3xcr Kunststoff-Additive by R. Gxc3xa4chter and H. Mxc3xcller, Hanser Verlag, Munich 1990, or DE-A-29 01 774.
Other additives which may be incorporated in the TPU are thermoplastics, for example, polycarbonates and acrylonitrile/butadiene/styrene terpolymers, particularly ABS. Other elastomers such as rubber, ethylene/vinyl acetate copolymers, styrene/butadiene copolymers and other TPUs may also be used. Moreover, commercial plasticisers such as phosphates, phthalates, adipates, sebacates and alkylsulfonates are suitable for incorporation.
The preparation process according to the invention is carried out as follows:
Components A) and B) are heated separately, preferably in a heat exchanger, to a temperature between 170xc2x0 and 250xc2x0 C. and metered in liquid form simultaneously and continuously into a static mixer preferably with a length/diameter ratio of 5:1 to 20:1, most preferably 8:1 to 14:1.
There the components are mixed homogeneously at a shear rate of 500 to 50,000 secxe2x88x921 and reacted. Homogenous mixing within the meaning of the invention means that the concentration distribution of the components and of the reaction product in the mixture has a relative standard deviation of less than 5%. The residence time in the static mixer is a maximum of 1 second.
The static mixer is insulated and heated preferably to 200xc2x0 to 260xc2x0 C. Static mixers which may be used according to the invention are mentioned in Chem.-Ing. Techn. 52, no. 4, page 285 to 291 and in xe2x80x9cMischen von Kunststoff und Kautschukproduktenxe2x80x9d, VDI-Verlag, Dxc3xcsseldorf 1993. Examples include SMX static mixers from Sulzer.
According to the invention, a conversion of  greater than 90%, based on the starting component A) is obtained in this static mixer, and the reaction mixture has a temperature of  greater than 240xc2x0 C. and  less than 350xc2x0 C. on leaving the static mixer.
In a particular embodiment, the reaction mixture is metered, optionally via a second static mixer, directly into a continuously operating kneader and/or extruder (e.g. a ZSK twin-screw kneader) where additional auxiliaries may be incorporated in the TPU at temperatures from 120 to 250xc2x0 C.
In the second static mixer, if present, a reaction takes place according to the invention only to a very small degree ( less than 10% based on the starting component A)). Pelletising is carried out at the end of the extruder.
The TPU prepared by the process according to the invention may be processed to injection moulded articles, extruded articles, particularly hot melt films, to coating compounds or sintered types and to readily melting coextrusion types such as, e.g., laminating, calendering and powder-slush types. Having good homogeneity, it is characterised mainly by a low softening temperature, as are the moulded articles produced therefrom.